Field emission guns have much higher brightness than thermionic electron guns which have been employed for many years. Therefore, when a field emission gun is used in an electron microscope or other instrument utilizing an electron beam, the performance of the instrument is greatly enhanced.
A conventional field emission gun is shown in FIG. 1, where a hairpin filament 2 is mounted in an evacuated vessel 1. The gun includes a needle-like emitter 3 spot-welded to the filament 2, a first control electrode 4 (also known as an extraction electrode), and a second control electrode 5. An accelerating voltage source 6 produces an output voltage V.sub.0 that is applied between the emitter 3 and the second control electrode 5. Since the second control electrode 5 is usually maintained at ground potential, the emitter 3 is held at a high negative potential. The output voltage V.sub.0 is the accelerating voltage of the gun. An extracting voltage source 7 generates a voltage V.sub.1 that is applied between the emitter 3 and the first control electrode 4, in order to create a potential difference for inducing field emission at the tip of the emitter 3. The opposing surfaces of the first control electrode 4 and the second control electrode 5 are so machined that they form a Butler electrostatic lens for focusing the electron beam 9 along the optical axis 8 of the electron optics. This electrostatic lens focuses the electron beam which diverges from a virtual electron beam-emitting point, known as a crossover point, formed near the tip of the emitter. The electrostatic lens forms an image of the crossover point at a point P lying on the optical axis 8. Let S.sub.1 be the distance between the emitter 3 and the first control electrode 4, d be the distance between the first control electrode 4 and the second control electrode 5, and S.sub.2 be the distance between the second control electrode 5 and the point P at which the image of the crossover point is formed.
In FIG. 2, S.sub.2 /d is plotted against V.sub.0 /V.sub.1, and in which S.sub.1 /d is a parameter. The vertical axis of the graph, or S.sub.2 /d, is the distance S.sub.2 normalized by the distance d, while the horizontal axis, or V.sub.0 /V.sub.1, is the ratio of the voltage V.sub.0 to the voltage V.sub.1. The parameter S.sub.1 /d is the distance S.sub.1 normalized by the distance d. It can be seen from the graph of FIG. 2 that when the accelerating voltage V.sub.0 is varied, the position P of the crossover point image shifts.
When the position P moves, the path followed by the electron beam in the electron optical system varies and so various adjustments are necessary. For example, in a scanning-type electron microscope or the like, it is often necessary to vary the voltage for accelerating the electron beam, depending on the kind of the specimen to be observed. If the position P is shifted when the accelerating voltage is changed, then the observed image that is obtained while a scan is being made is defocused, or the brightness is greatly changed. As a result, it is necessary to readjust the excitation of each electron lens in the electron optical system.
FIG. 3 shows one example of an apparatus which is designed to prevent the position P of the crossover point image from shifting. This apparatus uses a first control electrode 10, a second control electrode 11, and a third control electrode 12. In the apparatus shown in FIG. 1, the first control electrode 4 acts as an extraction electrode for placing the tip of the emitter in a strong electric field. The electrode 4 also serves as one electrode of an electrostatic lens. On the other hand, the second control electrode 10 and the third control electrode 11 shown in FIG. 3 act only as the electrodes of an electrostatic lens. The apparatus shown in FIG. 3 further includes an extraction voltage source 7 for creating a potential difference between the emitter 3 and the first control electrode 10, and an accelerating voltage source 6 for creating a potential difference between the emitter 3 and the third control electrode 12. It is necessary to provide a first lens voltage source 13 for creating a potential difference of V.sub.2 between the emitter 3 and the second control electrode 11. This structure permits the magnitude of the electrostatic lens to be changed while the extraction voltage V.sub.1 is maintained constant. Therefore, when the accelerating voltage V.sub.0 is varied, the magnitude of the electrostatic lens is adjusted by changing the potential V.sub.2 applied between the emitter 3 and the second control electrode 11. Thus, the position P of the crossover point image is prevented from shifting. As an example, a potential of -100 KV, a potential of -100 to -90 KV, and a potential of about -100 KV are applied to the emitter, the first control electrode, and the second control electrode, respectively.
The apparatus shown in FIG. 3 can be used in an apparatus where an electron beam is accelerated by a voltage less than about 100 KV but it cannot be employed as it is in an apparatus using an acceleration voltage of about 200 KV or more, since it is difficult to create a potential difference of about 100 KV or more either between the second control electrode 11 and the third control electrode 12 which form an electrostatic lens or between the first control electrode 10 and the second control electrode 11 because of the possibility of electric discharge or other phenomenon. These techniques are disclosed in U.S. Pat. No. 3,946,268.
In an attempt to solve these problems, an apparatus shown in FIG. 4 has been proposed. This apparatus includes an accelerating voltage source 14 which can produce up to 200 KV. An outside vessel 16 is mounted around the side wall 15 of an evacuated vessel 1. The side wall 15 consists of a cylinder made of an insulating material. The space between the outside vessel 1 and the cylindrical side wall 15 is filled with an insulating gas, such as Freon gas. This insulating gas maintains the insulation between the terminals mounted outside the insulating side wall 15. A first control electrode 10, a second control electrode 11, and a third control electrode 12 are disposed inside the evacuated vessel 1 in the same manner as in the apparatus shown in FIG. 3. However, the third electrode 12 is kept not at ground potential but at about -140 KV by the output V.sub.3 from a second lens voltage source 17. Five accelerating electrodes 18a, 18b, 18c, 18d, and 18e are mounted below the third electrode -2 and supplied with potentials of -125 KV, -100 KV, -75 KV, -50 KV, and -25 KV, respectively, for example, via voltage-dividing resistors 19a, 19b, 19c, 19d, 19e, and 19f which are connected in series between the output of the accelerating voltage source 14 and ground potential. These techniques are disclosed in U.S. Pat. No. 4,160,905. These accelerating electrodes have holes through which the electron beam passes. Since the diameter of these holes is much larger than the diameter of the hole formed in the second control electrode 11 in the same way as the first control electrode 10, the effects of the electrostatic lens formed by the accelerating electrodes on the electron beam can be almost neglected.
In the apparatus shown in FIG. 4, the ratio of the potential at the second control electrode 11 with respect to the emitter to the potential at the third control electrode 12 with respect to the emitter is kept constant. Therefore, as can be seen from the description made in connection with FIG. 2, the position of the crossover point image is prevented from shifting irrespective of changes in the accelerating voltage. However, this apparatus needs the second lens voltage source 17 between the emitter 3 and the third control electrode 12, unlike the apparatus shown in FIG. 3. The output voltage from the second lens voltage source 17 is considerably higher than the output, of the order of 10 KV, from the emitter voltage source. This makes the voltage sources of the instrument bulky and complex.
The electrodes of the electron gun are connected with the voltage sources by an insulated cable 20. Since the potential at a core 21 connected with the third control electrode 12 differs from the potential at the other cores by as much as 60 KV, the diameter of the cable 20 is very large. This renders the apparatus larger.
Field-dispersing rings 22a, 22b, 22c, 22d, and 22e which are mounted outside the accelerating electrodes, respectively, are kept at the same potentials as their respective accelerating electrodes mounted inside the evacuated vessel. These rings act to prevent the electric field distribution inside the outside vessel 16 from concentrating at certain locations.